commit	6f0d015029eab07f28f2fd09811aa8e4dea7010c	[log] [tgz]
author	Xin Li <[email protected]>	Sat Feb 20 13:53:09 2021 +0000
committer	Automerger Merge Worker <[email protected]>	Sat Feb 20 13:53:09 2021 +0000
tree	6dbbba641a2d90f06fdc582379a9d43be23e930f
parent	d324380d73c00a3273b80e1992aa69b15799c1ad [diff]
parent	e362c7d55fee88794fe66b261fcfa656aa4b226d [diff]

tree: 6dbbba641a2d90f06fdc582379a9d43be23e930f

README.md

rust-fnv

An implementation of the Fowler–Noll–Vo hash function.

Read the documentation

About

The FNV hash function is a custom Hasher implementation that is more efficient for smaller hash keys.

The Rust FAQ states that while the default Hasher implementation, SipHash, is good in many cases, it is notably slower than other algorithms with short keys, such as when you have a map of integers to other values. In cases like these, FNV is demonstrably faster.

Its disadvantages are that it performs badly on larger inputs, and provides no protection against collision attacks, where a malicious user can craft specific keys designed to slow a hasher down. Thus, it is important to profile your program to ensure that you are using small hash keys, and be certain that your program could not be exposed to malicious inputs (including being a networked server).

The Rust compiler itself uses FNV, as it is not worried about denial-of-service attacks, and can assume that its inputs are going to be small—a perfect use case for FNV.

Usage

To include this crate in your program, add the following to your Cargo.toml:

[dependencies]
fnv = "1.0.3"

Using FNV in a HashMap

The FnvHashMap type alias is the easiest way to use the standard library’s HashMap with FNV.

use fnv::FnvHashMap;

let mut map = FnvHashMap::default();
map.insert(1, "one");
map.insert(2, "two");

map = FnvHashMap::with_capacity_and_hasher(10, Default::default());
map.insert(1, "one");
map.insert(2, "two");

Note, the standard library’s HashMap::new and HashMap::with_capacity are only implemented for the RandomState hasher, so using Default to get the hasher is the next best option.

Using FNV in a HashSet

Similarly, FnvHashSet is a type alias for the standard library’s HashSet with FNV.

use fnv::FnvHashSet;

let mut set = FnvHashSet::default();
set.insert(1);
set.insert(2);

set = FnvHashSet::with_capacity_and_hasher(10, Default::default());
set.insert(1);
set.insert(2);